Here's the deal as I understand it now and it MAY not apply to every power supply out there, so proceed at your own risk!
Most modern and quality power supplies have a yellow sense strand for the 12 volt side of life and a red sense strand for the 5 volt side of life and a orange sense strand for the 3.3 volt side of PC powering life.
Some of the older or cheaper power supplies only have a sense strand for the 5 volts and the 3.3 volts and what you do with those power supplies is put the resistor on the 5 volt sense strand and it ups the 5 volts and the 12 volts to the same percentage of original value.
Then, if you are so cheap like me and find an old PC at the dumpster and scrap out a power supply... oh this looks promising!... it says CompUSA on it and 150 watts!
That type only has a sense lead for the 3.3 volts and I was told you were out of luck on modding the voltages on that one, but I tried it and found the 5 and 12 were just slaved to the 3.3 on that power supply at least.
The moral of the story... if any!... never underestimate what you can so easily smoke test! :Q